ppm means parts per million. To answer straightly to your question
50 to 70 mg of ascorbic acid for each one kg of the overall ingredients taken as reference, presumably those explicitly mentioned or at least discussed at that point.
Since you are not dealing with analysis you can approximate and add that amount to one kg of ingredients, as for one kg plus some mg is still one kg, at least in the kitchen.
If you use less than one kg of flour then you simply go by usual proportion. For 700 g choosing the higher dose of 70 ppm means 50 mg, for instance.
Just be sure to what ppm refers to, weight of just flour(s) or flour plus water to handle it.
This is just to clarify to a broader audience, not necessarily because of importance in the kitchen, what ppm are and how can be used.
ppm is parts per million by weight. Often it is treated as mg/l just because of working with dilute solutions in water. As for diluted water based solutions have a density very close to 1, in this case 1 ppm is indeed equal to 1 mg in 1 liter of solution. That is.
So if you would have to add ascorbic acid at mg scale to water as a base for, say, a syrup, than 1 ppm is indeed 1 mg/l.
As soon you add spoons, than ppm won't makes operatively sense and one would say 100 g for liter of water.
Finally one can refer to volumes or mixed quantities but it should be explicitly stated. Is an issue present in research as well, for instance when solutions prepared in solvent with different densities are mistakenly treated without thinking that mg/l is now different than ppm independently of concentration.